Apple and Celery Slice

It took me a long time to find out what exactly a "slice" is. After some Google research it still seemed kind of nebulous. A bar cookie? A cake type thing? A biscuit type thing? Seems like it's any kind of dessert/sweet food that has a crust, is baked in a rectangle, and is cut into small rectangles to serve. We don't have anything called a slice in the US so I'm just trying to figure it out.

2 1/2 cups white wheat flour, whole wheat flour, or whole wheat pastry flour
1 teaspoon salt
1/3 cup butter
1/3 cup cream cheese
1 egg yolk
1/2 cup milk or nut milk
2-3 apples, cored and sliced thinly
1 stick of celery, sliced thinly
2 Tbls. sugar
2 Tbls. honey
1 teaspoon ground nutmeg
1 tsp fennel seeds (or 1/2 seeds, 1/2 leaves)
1 tsp. arrowroot powder
1 egg white

Preheat oven to 375 F.

In a large mixing bowl stir together the flour and the salt. Cut in butter and cream cheese till the mixture resembles coarse crumbs. Lightly beat egg yolk in a measuring cup. Add milk to make 2/3 cup liquid total; mix well.  Divide dough in half.  

On a floured surface, roll half the dough thinly to a large square; fit into and up sides of an 8 x 8 inch baking pan.  Trim excess.

Stir together the apples, celery, honey, sugar, nutmeg, fennel, and arrowroot powder. Place in the crust. 

Roll remaining dough to a large, thin square; place over apples. Trim excess and seal edges.  Beat egg white until frothy, and brush on top of pastry. Cut slits in the top for steam to escape. 

 Bake at 375 degrees F for about 50 minutes or until golden brown.